1. Field of the Invention
The present invention relates to a fuel cell system and more particularly to a fuel cell system using a proton exchange membrane as an electrolyte.
2. Description of the Prior Art
A proton exchange membrane fuel cell comprises a proton exchange membrane (PEM) between two electrodes that is a cathode to which an oxidizing gas is supplied and an anode to which fuel gas is supplied. PEM acts as an electrolyte and transports therethrough hydrogen ions obtained at the anode of the fuel cell toward the cathode in the form of proton (H.sup.+). Each of the electrodes comprises a catalyst layer deposited on a porous base member through which the reactant gas is supplied. Mounted externally of each electrode is a separator or connector plate with grooves permitting the reactant gas to be introduced into the electrode at a constant flow rate. Excess gas which has not been consumed by the fuel cell reaction is exhausted to the open air through the grooved separator. The electricity generated by the energy conversion reaction at the anode is collected at the electrode porous base member and transported to the outside of the fuel cell system through the separator. In actual application, the system includes a plurality of fuel cells which are stacked in series with the separator being interposed between adjacent fuel cells.
Since the fuel cell generates heat in correspondence to the electric power generated, a fuel cell stack usually includes cooling plates between fuel cells at predetermined intervals. Each cooling plate has a passage of a cooling medium such as air and water to prevent overheating of fuel cells in operation.
Protons are hydrated in transfer through the PEM electrolyte, so that the PEM tends to become dehydrated as the fuel cell reaction proceeds. The PEM must always be properly humidified to prevent decrease of ion-conductivity and energy conversion efficiency. In the conventional designs, hydrogen gas is humidified by suitable means which, in turn, humidify the PEM when it is supplied to the anode.
Various attempts have also been proposed to humidify the air supplied to the cathode. Since the cathode of the fuel cell operates at 80.degree. C., for example, the air of a normal temperature should be preheated by a humidifier so that its saturated vapor becomes consistent with the ambient vapor condition of the cathode. Such a humidifier that is required to have both a water supplying function and an air preheating function can not be simple in construction.
In Japanese patent un-examined publication No. 7-14599, there is provided a water injection nozzle to inject a necessary quantity of water into an air introducing pipe through which air is supplied to the cathode of the PEM fuel cell. Since the nozzle is located upstream of a compressor, liquid water injected from the nozzle is evaporated by the heat generated by the compressor. Thus, the cathode is humidified by vapor, not by liquid water.
In the fuel cell system of Japanese patent un-examined publication No. 9-266004, a discharge gas from the anode containing hydrogen gas which has not been consumed during the anodic reaction is introduced into the cathode where the unconsumed hydrogen gas in the discharge gas is combusted with oxygen to generate water, which well humidifies the PEM electrolyte. In this system, there is no need to install a humidifier for humidifying the air supplied to the cathode.
During operation of the fuel cell system, electrons produced at the anode move to the cathode where they react with oxygen in the air or any other oxidizing gas supplied thereto to produce water. Accordingly, in accordance with the conventional practice in the art, there is a greater need to humidify hydrogen gas to be supplied to the anode, than at the cathode where supply of water is at least partially self-sustaining.
As a result of the inventors' repeated tests and investigation, however, it has been found that water produced at the cathode permeates through the PEM electrolyte toward the anode, which makes it unnecessary to humidify hydrogen gas to be supplied to the anode. On the other hand, the quantity of water in the PEM electrolyte at the cathode tends to be decreased by the air flow to the cathode. Such finding is contradictory to the conventional understanding and has been first recognized by the present inventors.